Among fan motors for vehicle, a fan motor for vehicle using a permanent magnet as a field (DC motor) is known. In this type of fan motor for vehicle, generally even number of field magnets are fixed inside a motor yoke so that the field magnets are opposed to each other, and an armature provided with windings is disposed more internally than the field magnets.
In this case, for example, even if four field magnets are used to form four electrodes, because two brushes for commutation (a positive brush and a negative brush) can be incorporated, or for other reasons, the armature comprises slots in which wave windings are respectively provided between teeth. The windings of the armature are respectively electrically connected to odd number of segments of a commutator, wherein the number of segments is determined in correspondence to the number of slots of the armature. The positive brush and the negative brush are press-contacted to the segments of the commutator. Accordingly, electricity is supplied to the respective slots of the armature (commutation is carried out) via the brushes and the segments.
In such a DC motor, since the brushes are press-contacted to the respective segments of the commutator, which segments are provided in correspondence to the respective slots of the armature, so as to supply electricity (commutate), as the armature rotates, coils in a short circuit state occur, wherein the coils are concurrently made to have the same potential and the occurrence positions are primarily determined (for example, the occurrence positions are four positions in the case where four electrodes and two brushes are used).
The short circuit state of the coils is a state of windings (coils) 40 wound on a laminated core of the armature, as shown in FIG. 4. For example, when each of a positive brush 42 and a negative brush 44 contacts across two mutually adjacent segments 46 of the commutator, the respective two segments 46 (segments of Nos. 6 and 7, and segments of Nos. 12 and 1) have the same potential and current does not flow therethrough, thus resulting in the short circuit state. In this case, coils A and B connected to the respective two segments 46 of the commutator are referred to as the coils in the short circuit state. Such coils in the short circuit state serially occur, while the occurrence positions are primarily determined depending upon the positional relation between the segments 46 of the commutator, and the positive brush 42 and the negative brush 44. In FIG. 4, the flow of current is schematically shown by arrows.
In this type of conventional fan motor for vehicle, since the pitches of the segments 46 of the commutator are regular, timing when the coil in the above-described short circuit state occurs, wherein the coil in the short circuit state serially occurs every time each of the positive brush 42 and the negative brush 44 contacts across two segments 46 of the commutator, and timing when the coil is released from the short circuit state (timing of switching from the short circuit state to a conductive state) of one brush are coincident with those of the other brush. This has been a cause that a value of electric current supplied to the respective slots of the armature changes rapidly in a short time and falls into ununiform disorder (variation width of current waveform changes considerably in waves).
In this manner, the timing when the coil in the above-described short circuit state occurs, wherein the coil in the short circuit state serially occurs every time each of the positive brush 42 and the negative brush 44 contacts across two segments of the commutator, and the timing when the coil is released from the short circuit state of one brush are coincident with those of the other brush, and thus, the variation width of current waveform widens. This results in vibration of the motor, variation of rotating torque, and generation of noise. There has been eagerness for countermeasures therefor.
In view of the above facts, it is an object of the present invention to obtain an electric rotating machine and a fan motor for vehicle, wherein variation width of current waveform resulting from occurrence of coils in a short circuit state is narrowed to reduce the vibration and the noise.